Silicone compositions have long been used for rendering surfaces nonadherent to materials which would normally adhere thereto. For a long time, it was necessary that these silicone coatings be applied as a dispersion within a solvent in order to control the viscosity of the coating material so as to be suitable for coating applications. However, although the solvent aids in the application of the coating, it is a highly inefficient process inasmuch as the solvent must thereafter be evaporated. The evaporation of solvents requires large expenditures of energy and pollution control requirements mandate that solvent vapors be prevented from escaping into the air. Removal and recovery of all the solvent entails considerable expenditure for apparatus and energy.
Thus, it has been noted that there is a need to provide a solventless coating composition which will, however, remain easy to apply to the substrate. Such solventless coating compositions are sometimes referred to as "100% solids" compositions. The absence of solvent in such compositions both lowers the amount of energy required to effect a cure and eliminates the need for expensive pollution abatement equipment. The composition of the present invention is a low viscosity polydiorganosiloxane fluid which can readily be coated onto a substrate by standard methods such as blade, press, brush or gravure and which will thermally cure onto the substrate to form a nonadherent surface.
Release coatings are useful for many applications whenever it is necessary to provide a surface or material which is relatively nonadherent to other materials which would normally adhere thereto. Silicone paper release compositions are widely used as coatings which release pressure-sensitive adhesives for labels, decorative laminates, transfer tapes, etc. Silicone release coatings on paper, polyethylene, Mylar, and other such substrates are also useful as non-stick surfaces for food handling and industrial packaging applications.
For example, when labels are coated with an adhesive, it is desirable that the paper backing be easily peeled away from the label when it is ready for use, yet the adhesive quality of the label should not be derogated by the fact that it has been peeled away from the substrate upon which it was stored. The same principle applies to certain types of adhesive tapes which come in rolls. It is necessary that the tape unroll easily and still maintain its adhesive characteristics. This can be accomplished by coating the nonadhesive side of the tape with a silicone release composition which will come into contact with the adhesive as the roll of tape is manufactured.
Two problems previously encountered with silicone compositions of the above-described type are related to premature gelation of the composition. Inasmuch as a typical composition can be made up of a vinyl chainstopped base polymer, a crosslinking agent and a platinum metal catalyst, premature curing may occur over a length of time resulting in a product with unsatisfactory shelf-life. Furthermore, when the composition is ready for use, it is desirable that the catalyzed polymer composition have a sufficiently long pot-life to enable one skilled in the art to carry out the coating application. Thus, the coating composition of the present invention is provided with an inhibitor which effectively retards the hydrosilation addition cure reaction of these compositions at ambient temperature, but which does not retard the cure at elevated temperature.
An example of the use of an ethylenically unsaturated isocyanurate as an inhibitor for an addition curable room temperature vulcanizable organopolysiloxane elastomer may be found in U.S. Pat. No. 3,882,083 (Berger et al.) which is assigned to the same assignee as the present invention and is hereby incorporated by reference.
Silicone paper release coatings are low viscosity materials which are cured by SiH-olefin hydrosilation reactions catalyzed by platinum-metal compounds and are well known products. In order to prevent too rapid gelation of the catalyzed bath during the coating procedure, an inhibitor is added which prevents the crosslinking or curing reaction from taking place at low (i.e. room) temperatures, but which does not impede curing at higher (i.e. oven) temperature. Inhibitors which have been used in solventless paper release products include vinyl acetate, diallylmaleate and triallylisocyanurate (TAIC). The use of diallylmaleate as an inhibitor for paper release coatings is disclosed in U.S. Pat. No. 4,256,870 (Eckberg) which is hereby incorporated by reference. Use of vinyl acetate is objectionable as an inhibitor due to problems with toxicity and volatility. TAIC can be a satisfactory inhibitor, but it is not sufficiently soluble in solvent-free silicone fluids to permit its use at levels higher than approximately 2500 ppm. For these reasons it has been felt important that an improved inhibitor be discovered. The present invention discloses the discovery that dialkylacetylenedicarboxylates will function as effective inhibitors in solventless paper release systems.
Certain other acetylene derivatives have been recognized as effective inhibitors for platinum-catalyzed organopolysiloxane compositions, as described in U.S. Pat. No. 3,445,420. However, the disclosure relating to acetylenic inhibitors specifically excludes any acetylene compounds possessing nitrogen, carboxyl, phosphorus, mercapto, and carbonyl groups alpha to the aliphatically unsaturated carbon atoms, thus making the disclosure in U.S. Pat. No. 3,445,420 significantly different than the present disclosure. The discovery disclosed herein, i.e. inhibition by acetylenedicarboxylates ROOCC.tbd.CCOOR, wherein R is a monovalent hydrocarbon radical, clearly does not fall within the scope of the previously patented compositions.
Therefore, a primary object of the present invention is to provide a solventless composition which will render surfaces non-adherent to materials which would normally adhere thereto.
Another object of the present invention is to provide a solventless coating composition which will cure to a coating having satisfactory smear, rub-off, migration and release characteristics.
Another object of the present invention is to provide a solventless coating composition which has a satisfactory pot-life or bath-life in its catalyzed form when it is ready for use.
Another object of the present invention is to provide a method of rendering materials nonadherent to other materials which would normally adhere thereto.
Another object is to provide articles of manufacture having nonadherent surfaces by coating the surfaces with the release compositions of the present invention.
Another object of the present invention is to provide a release coating composition having a one-part inhibitor which effectively retards the catalyzed crosslinked reaction at temperatures below the heat cure temperature of these compositions and to provide solventless silicone compositions which are particularly well suited for paper release applications.
These and other objects will become apparent to those skilled in the art upon consideration of the present specification, examples and claims.